1. Technical Field
The embodiments herein generally relate to wireless communications, and, more particularly, to a method and apparatus for Doppler frequency estimation for Digital Video Broadcasting-Handheld (DVB-H) systems.
2. Description of the Related Art
Doppler frequency is of great importance in mobile communication systems. It relates directly to the velocity of the vehicle through the equation: fd=v/λ=(v/c)*fc, where fd is the Doppler frequency, v is the relative velocity between the transmitter and the receiver, λ is the wavelength of the transmitted signal, c is the speed of light in free space and fc is the carrier frequency. Knowledge of the Doppler frequency enables one to adapt the bandwidth of the channel estimation filter and in turn enhance the demodulator performance. In DVB-H systems, time domain interpolation of the scattered pilots to produce a 1 out of 3 estimate for the channel can be adapted if the Doppler frequency is known. This is because in Rayleigh fading channels, it is observed that the power spectral density of one bin in time has the common U-shaped power spectral density with maximum frequency of fd.
Different methods have been proposed to estimate Doppler frequency for DVB-H systems. These methods include, for example, the Level Crossing Rate (LCR) method, the covariance based method, and the autocorrelation method. For an example, in the autocorrelation method, the autocorrelation of the in-phase and quadrature-phase components of the received signal may be given by:cr(τ)=real(E(r(k)r*(k+τ)))=(Ω/2)J0(2πfdτ)+(N0/2)δ(τ)  (1)where r(k) is the received signal at sub-carrier index k, Ω/2 is the variance of the received signal, J0 is the zeroth-order Bessel function of the first kind, and fd is the Doppler frequency.
In actual cases, the δ-function is replaced by a sinc function with certain noise bandwidth. A good estimate of the Doppler frequency can be obtained by calculating the autocorrelation of the received signal if the noise term ((N0/2) δ(τ)) in equation (1) is neglected.
However, if the correlation function of a received symbol and itself is calculated, the noise would correlate with itself, and thus, the noise term in equation (1) would dominate. To overcome this, as it is well-known that in DVB-Terrestrial/Handheld (T/H), the structure of the pilots repeats each four symbols, the correlation between the current symbol and a stored version of the symbol that is four symbols back in time is calculated. Thus, in this case the noise would not correlate and the noise term can be neglected.
However, as equation (1) of the autocorrelation method is dependent on the received signal variance Ω/2, accurate calculation of the received signal variance requires saving the history of the received signal for a long time. Saving the history of the received signal for a long time is not accepted from a hardware implementation perspective. To augment this, a ratio between c4 and c8 is calculated, where cx is the correlation coefficients between the current symbol and the symbol that is x-symbols back in time. The ratio c4/c8 may be given by:c8/c4=J0(2πfd*8Ts)/J0(2πfd*4Ts)  (2)
It can be observed that in equation (2), fd is the only unknown term. Thus, by using the relation of equation (2), a good estimate of fd by using a look-up table of the Bessel function values is obtained.
Alternatively, a ratio c4/c0 instead of c8/c4 can be determined to save the buffering memory. However, in determining c0, which is the correlation of the received symbol with itself, a noisy term is generated. The correlation ratio c4/c0 may be given by:c4/c0=((Ω/2)J0(2πfd*4Ts))/((Ω/2)+(N0/2))  (3)
Equation (3) can be approximated to c4/c0=J0(2πfd*4Ts) in high signal to noise ratio (SNR) conditions. However, equation (3) gives a very bad result in low SNR conditions making the estimation method SNR dependent. Therefore, the correlation ratio c8/c4 is used, as the same is not dependent on the SNR.
However, the estimation of the Doppler frequency (fd) using equation (2) is appropriate in circumstances where there is a one-to-one mapping between the determined ratio (c8/c4) and the Doppler frequency (fd). This technique fails when two values of fd map to the same ratio. For example, for 8K mode G1/4 where Ts=1120 μs, the ratio c8/c4 is unique for each Doppler frequency for frequencies up to fd=92 Hz, however, for frequencies above fd=92 Hz some Doppler frequencies give c8/c4 ratios similar to those below fd=92 Hz. Further, for frequencies above fd=120 Hz some ratios repeat again as illustrated in FIG. 1A. Further, as illustrated in the example of FIG. 1A, an ambiguity is encountered when scattered pilots are used to estimate Doppler frequencies above fd=120 Hz. FIG. 1A is a graphical representation of a correlation ratio (c8/c4) curve for scattered pilots at different Doppler frequencies for 8K mode G1/4 where Ts=1120 μs. As illustrated in FIG. 1A, the curve 100 is not unique for the corresponding Doppler frequency. Further, it can be seen that there is a many-to-one mapping between the correlation ratio and the corresponding Doppler frequency.
Thus, to reach high Doppler frequencies as large as 200 Hz, for 8K mode G1/4 where Ts=1120 μs, this technique of using the scattered pilots fails as Doppler frequencies above fd=120 Hz cannot be distinguished from Doppler frequencies up to fd=92 Hz, and above fd=92 Hz and below 120 Hz. In other words, the technique of using scattered pilots generally fails for Doppler frequencies greater than 92 Hz (i.e., it will not distinguish Doppler frequencies higher than 92 Hz from those lower than 92 Hz). The maximum limit of this method can be increased from 92 Hz to 120 Hz if the sign of c4 is used as an extra piece of information (in addition to the ratio of c8/c4). The limitation on the maximum Doppler frequency comes from the large oscillations of the Bessel function in the range of Doppler frequencies of interest which results in many-to-one mapping. The zeroth-order Bessel function of the first kind, J0(2πfdτ), wherein τ is the difference in time, has the property that for a certain τ, the rate of oscillations increase as the maximum allowed Doppler frequency increases. For a certain maximum Doppler frequency, the oscillations increase as τ increases. Therefore, as the maximum Doppler frequency to be estimated is to be increased, the difference in time is to be decreased.